Wireless communication systems are widely spread all over the world to provide various types of communication services such as voice or data. The wireless communication system is designed for the purpose of providing reliable communication to a plurality of users irrespective of their locations and mobility. However, a wireless channel has an abnormal characteristic such as a fading phenomenon caused by a path loss, noise, and multipath, an inter-symbol interference (ISI), a Doppler effect caused by mobility of a user equipment, etc. Therefore, various techniques have been developed to overcome the abnormal characteristic of the wireless channel and to increase reliability of wireless communication.
A multiple input multiple output (MIMO) scheme is used as a technique for supporting a reliable high-speed data service. The MIMO scheme uses multiple transmit antennas and multiple receive antennas to improve data transmission/reception efficiency. Examples of the MIMO scheme include spatial multiplexing, transmit diversity, beamforming, etc.
A MIMO channel matrix is formed by multiple receive antennas and multiple transmit antennas. A rank can be obtained from the MIMO channel matrix. The rank is the number of spatial layers. The rank may also be defined as the number of spatial streams that can be simultaneously transmitted by a transmitter. The rank is also referred to as a spatial multiplexing rate. If the number of transmit antennas is Nt and the number of receive antennas is Nr, a rank R satisfies R≦min {Nt, Nr}.
A wireless communication system requires a signal known to both a transmitter and a receiver to perform channel measurement, information demodulation, or the like. The signal known to both the transmitter and the receiver is referred to as a reference signal (RS). The RS may also be referred to as a pilot.
The receiver may estimate a channel between the transmitter and the receiver by using the RS, and may demodulate information by using the estimated channel. When a user equipment receives an RS transmitted by a base station, the user equipment may measure a channel by using the RS, and may feed back channel state information to the base station.
A signal transmitted from the transmitter experiences a channel corresponding to each transmit antenna or each spatial layer, and thus the RS may be transmitted for each transmit antenna or each spatial layer. If the RS is transmitted for each spatial layer, the RS may be transmitted after precoding. In this case, the receiver needs to know information on a frequency region in which the same precoding matrix is used.
Accordingly, there is a need to provide a method and apparatus for effectively transmitting a signal in a wireless communication system.